1. Field of the Invention
The present invention relates generally to a call connection between communication equipments using an ISDN packet switching mode service. More specifically, the invention relates to a method for selecting a called equipment.
2. Description of the Related Art
Conventionally, when a called equipment is to be selected in an ISDN packet switching mode, selection has been performed in response to call demand based on CCITT recommendation X.25, by setting a network address (for example, the address corresponding to NSAP address under OSI environment) of a destination end system connected to the called equipment in an address value field in a called address extending facility of a call demand packet under X.25. Such system has been disclosed in Japanese Unexamined Patent publication No. Heisei 2-97193 and Japanese Unexamined Patent Publication No. Heisei 3-88534 and its U.S. equivalent, U.S. Pat. No. 5,181,199.
It should be appreciated that, in this case, an ISDN network feeds the same content which is same as the address value facility in response to the call remand under X.25 according to CCITT recommendation Q.93 with mapping the content in the address value field in a called sub-address information factor of a call setup message as one of ISDN call control messages. Then, in the called equipment, judgement is made whether the call is designated to the own equipment or not on the basis of a network address set in the address value field in the call setup message.
However, the sub-address set in the incoming call sub-address information factor of the call setup message is originally for supplementing ISDN number (number of channel assigned for a terminal upon subscribing ISDN). Namely, the sub-address is an identifier for a plurality of equipments belonging an interface, to which the ISDN number is assigned. This sub-address is different from the network address as the identifier of the destination end system and rather a more physical address depending upon a sub-network. Accordingly, in the conventional method as set forth above, it is forced to use the network address having a logical meaning as the physical sub-address.
Therefore, in the called ISDN terminal (including a terminal adapter) under a multi-point connection environment, it becomes necessary to manage one or more network address groups presented in one terminal equipment as sub-addresses of own terminal equipment. Therefore, large amount of sub-address information has to be managed by the called terminal equipment.
Especially, when the called equipment under the multi-point connection environment serves as a router for other network environment, such as from ISDN to LAN or so forth, such router equipment has to manage huge amount of network addresses of other network environment connected therefrom.
Furthermore, when a plurality of protocol is present within the called ISDN terminal equipment, not only the sub-address information becomes huge, but also the protocol discrimination becomes necessary upon the called terminal section since formats of the protocols are differentiated in every network systems. Therefore, the processing load of the called ISDN terminal equipment is increased.